An apparatus of the generic kind is described in DE 100 43 600 A1, EP 1 060 301 B1 and EP 1 240 366 B1. The devices have a reactor housing, which is closed in gas-tight manner against the environment. There is a process chamber within the reactor housing. This has the form of a circular cylinder. The disk-shaped floor of the process chamber is formed by a multi-part susceptor consisting of graphite, which has on its side facing the process chamber a multiplicity of recesses arranged around the center, in which disk-shaped substrate holders are enclosed, on the surface of which in each case a substrate to be coated is supported. The substrate holders are driven in rotation, supported on a gas cushion by a gas flow. The susceptor that rests on a central pillar may likewise be rotated about the axis of symmetry of the process chamber. Above the susceptor that extends in a horizontal plane, there is a process chamber ceiling which likewise is made of graphite. In the center of the process chamber ceiling, there is a gas inlet element which is connected by feed lines to a gas mixing system by which outlet channels of the gas inlet element are supplied with a carrier gas and with process gases transported by the carrier gas. The process gases may be on the one hand an organometallic substance, e.g. TMGa, TMIn or TMAl. The other process gas is a hydride, e.g. arsine, phosphine or ammonia. Using these process gases, semiconductor layers are to be deposited on the substrate surface, which layers may consist of Ga, In, Al, P, As and N. Using the apparatus, there can be deposited not only III-V semiconductor layers, but also, by suitable choice of the starting materials, II-VI semiconductor layers. It is furthermore possible to dope the deposited semiconductor layers by the addition of suitable further highly diluted starting materials.
The process gases introduced into the center of the process chamber with the carrier gas flow through the process chamber in the horizontal direction parallel to the process chamber ceiling and the process chamber floor. The process chamber floor and the optionally also the process chamber ceiling are heated to a process temperature. This is effected by an RF heater. For this, there are, underneath the susceptor, windings of a water-cooled heating spiral. The process gases decompose pyrolytically into decomposition products on the hot surfaces and in particular on the hot substrate surface. The process is conducted in such a way that the growth of the semiconductor layers on the substrate takes place in a kinetically controlled temperature range, since in this temperature range, the highest crystal quality can be achieved. It cannot be avoided that during the process, a parasitic coating grows on the surface regions of the susceptor that surround the substrates, on the underside of the process chamber ceiling, and on a gas outlet element that forms the downstream wall of the process chamber.
The thermal treatment processes that take place in the apparatus under discussion require process temperatures of different magnitudes. In a generic method, a first layer of a first material is deposited on the substrate in a first process step. This is effected at a low process temperature, which can be for example in the range from 500° C. to 800° C. After one or more purge steps and optionally also further intermediate steps, a second process step is carried out for which the process temperature is significantly higher, e.g. is at least 1,000° C. In this process step, a layer of a second material is deposited onto the first layer or onto further intermediate layers. The gas outlet element that was coated with decomposition products in the first process step was neither cleaned nor changed in the intervening period between the first and second process steps. In a subsequent analysis of the layer deposited in the second process step, traces were found of the decomposition products of the process gases introduced into the process chamber in the first process step.
Using the generic apparatus, there can furthermore also be carried out a deposition method which begins with a process step that is carried out with a very high process temperature, e.g. 1,600° C. This process step may be a heat treatment step. If, in a previous deposition process on a substrate which was exchanged between steps, a growth step was carried out at a lower temperature, there was a parasitic coating of the gas outlet element. If the gas outlet element has not been replaced when the substrate was changed, a contaminated gas outlet element is present for the following coating operation during the heat treatment process. Also for this process procedure, a first process step at a low temperature in which growth of a layer takes place is followed by a second process step at a high process temperature. Here also, it was established by a subsequent analysis of the layers deposited that products of decomposition from the earlier process step had been incorporated into later-deposited layers.
Also belonging to the prior art is EP 0 449 821, which describes a CVD reactor in which a susceptor on which substrates to be coated are supported, is heated from beneath. The process chamber which is closed at the top by a cover is flowed through in the horizontal direction by process gas. In order to prevent a recirculation of the reaction gas at the gas outlet end, stabilizer vanes are provided, which define narrow channels through which the gas flows.
EP 0 252 667 describes a CVD reactor having a process chamber, the floor of which forms a rotatable susceptor on which a substrate is supported. A process gas flows through the process chamber in the horizontal direction, the process gas being urged from above in the direction of the susceptor by introduction of an additional inert gas. In this way, a laminar flow of gas is established over the substrate to be coated.
U.S. Pat. No. 5,891,251 describes a CVD reactor having a process chamber heated from beneath, through which flow takes place horizontally. Here also, additional inert gas streams are provided in order to influence the flow pattern of the process gas.
U.S. Pat. No. 5,951,772 describes a CVD reactor having a process chamber, the floor of which forms a susceptor, which is heated from below by a lamp. There is a shower-head type gas inlet element above the susceptor. The walls of the process chamber are heated. On a level lower than the susceptor, there is a gas outlet, in order to suck away process gas out of the process chamber by means of a pump.
US 2002/0000196 A1 describes a CVD reactor having a process chamber with a shower-head type gas inlet element and a substrate to be coated which is heated from below.
US 2003/0136365 A1 shows a gas outlet tube. It has an inner tube provided with a heating sleeve.
US 2005/0011441 A1 describes a CVD reactor having a susceptor on which a substrate to be coated is supported, the substrate being heated from below. A gas outlet is provided which lies at a lower level than the susceptor.
US 2006/0225649 A1 describes a CVD reactor with a shower-head type gas inlet element and a susceptor forming the floor of a process chamber, on which susceptor the substrate is supported. A gas outlet annulus is provided, this surrounding the susceptor at a spacing that leaves a gap.
US 2009/0044699 A1 tackles the problem of polymer formation in a gas outlet system. By means of a heater located there, the chemical bonds of the polymers are to be broken.
US 2009/0114155 A1 is concerned with a CVD reactor and a condensation trap located in the gas outlet flow, which trap can be heated.
JP 08078338 A describes a CVD reactor in which the substrate is held in a suspended state, so that it can be coated from both sides.
JP 10306375 A describes a special gas-mixing system for a CVD reactor, the valve arrangement of the system being selected so that pressure fluctuations within the process chamber are reduced.